It is a well-known disease with little treatment beyond prevention (mainly quarantine and avoiding contact with carriers) and it has a very high mortality rate, having been described as killing three of each four men in armies.[1]

Ser Barristan Selmy states that the bloody flux had been the bane of every army since the Dawn Age and that he has known the bloody flux to destroy whole armies when left to spread unchecked. Even hard men like the Second Sons are terrified of "mounting the pale mare," and they will drive out anyone suspected of having it without a moment’s hesitation.

A Storm of Swords

A Dance with Dragons

The bloody flux spreads through Astapor[4] and comes from there with the refugees that seek protection in Meereen. Warned about the dangers of letting them in, Daenerys decides to forbid their entrance, allowing instead that they camp outside the city.[1] However, the disease is already spreading and ravages both Meereen and its siegers from Yunkai and New Ghis.[5][6][7]

The first known case in Meereen was brought over by a dying Astapori rider atop a pale mare, which gave the plague its nickname.[1]